Frequency variation response circuit



Nov. 5, 1940. e. GUANELLA 2,220,098

FREQUENCY VARIATION RESPONSE CIRCUIT Filed June 1., 1938 2 Sheets-Sheet 1 15 m INVENTOR. I i, q guafav'e fiizanella Fl 7 I ATTORNEY.

Nov. 5, 1940. I e. GUANELLA 2,220,093

FREQUENCY VARIATION BESPQNSE CIRCUIT- 7 Filed June l, 1938 2 Sheets-Sheet 2 INVENTOR. FY81 :14 Qusfave Quanellq BY V ATTORNEY.

Patented Nov. 5, 1940 PATENT OFFICE FREQUENCY VARIATION RESPONSE CIRCUIT Gustav Guanella, Zurich, Switzerland, mum:

to Radio Patents Corporation, New York, N. a corporation of New York Application June 1, 1938, serial No. 211.158 In Switzerland July 11, 19:1

8 Claims.

frequency control (AFC) in a radio receiver,

transmitter, or otherelectrical system.

In the copending application, Ser. No. 146,555, filed June 5, 1937, by applicant and Max Lattmann, as a joint application, there is described an arrangement for generating a tuning responsive potential by utilizing the variations of the mutual phase angle between energies or potentials derived from difierent points of a resonant circuit or network and depending upon the sense and amount of detuning of the circuit relative to the frequency of an impressed carrier frequeJicy. The derived potentials of varying mutual phase angle are combined and rectified to obtain therefrom a tuning responsive direct potential varying both in polarity and magnitude in dependence upon the sense andin proportion to the amount of deviation of the tuning frequency of the circuit or network from the impressed signal frequency. This tune responsive potential may serve to operate a suitable indicator to assist in the operation and correct tuning adjustment oithe receiver or it may be applied to a tuning control device to eifect an automatic tuning or frequency control of a resonant circuit such as the oscillatory circuit of a self-excited oscillator such as the local oscillator in a superheterodyne receiver.

An object of the present invention is to provide an' improved discriminating circuit of the above general type which insures increased efilciency and stability compared with the known arrangements serving for a similar purpose.

Further objects and aspects of the invention will become more apparent from the following detailed description taken with reference to the accompanying drawings forming part of this specification and wherein;

Figures 1 to 3 show resonant networks suited for the purpose of the invention,

Figures 4 to 8 show practical embodiments of discriminating circuits employing networks of the type according to the preceding figures, l

Figures 9, 11 and 13 show further resonant networks suited for the purposes of the invention, and

Figures 10, 12 and 14 illustrate. discriminating systems embodying networks of the type shown in Figures 9, 11 and 13, respectively.

Similar. reference characters denote similar parts throughout the diflerent views of the drawings.

Referring to Figure 1. there is shown a bandpass filter comprising primary and secondary tuned circuits coupled with each other. The primary circuit in the example illustrated comprises an inductance IQ shunted by a capacity II which may be fixed or variable as shown. The secondary circuit similarly comprises an inductance I2 shunted by a capacity 13. The primary and secondary circuits are coupled through series 1 coupling reactances such as inductances l4 and I5 inserted in the upper and lower connecting leads as shown in the example illustrated. As is well known, according to the theory of electric filters, the voltage ea developed across the secondary circuit, terminals c-d, has a phase which differs from the phase of the primary voltage 21 applied to terminals aF-b by 90 if the medium or central frequency of the response characteristic of the filter corresponds to the frequency of 25 the impressed input potential. This phase difference deviates in either direction from 90 in dependence upon the sense and amount of deviation of the impressed frequency relative to the medium or central frequency of the filter response chars to acteristic, these phase variations comprising a range from 0 to 180. By comparing the phases of the potentials c1 and er, a tuning responsive or control potential is generated as will be described in detail hereafter.

In place of the coupling shown in Figure l, the circuits ill, Hand i2, 13 may be coupled by a small inductive or capacitative impedance such as an inductance coil l6 forming a common element of the primary and secondary inductances 40 i0 and 12 as shown in Figure 2. Alternatively, the common coupling inductance I 6 may be connected in series and symmetrically between a pair of condensers II and 13', respectively, replacing the condensers II and II, as shown in Figure 3. Furthermore, thacircuits I I, II and I2,

I3 may be inductively coupled to form a bandpass filter of well known construction as shown in Figure 6.

In all circuits of the type according to Figures 2 and 3, and similar arrangements, the output voltage ez has a phase relative to the input voltage e1 which varies from a normal relative phase .angle of at resonance condition in either direction accordingto the sense and in proportion 56 'and I2,

to the amount of deviation of the tuning of the circuit in respect to the frequency of an impressed carrier wave. Similar phase variations occur if the inductances l0 and i3 are directly coupled with each other or if a mixed coupling is employed.

Referring to Figure 4, there is shown a frequency discriminator or tune detecting system embodying a band-pass filter of the type accord-, ing to Figure 1. There is provided according to the invention for the comparison of the phases of the input and output voltages c1 and e: a rectifier or modulator bridge comprising four rectifier elements I9, 20, 2| and 22 connected in series in like sense to form a closed circuit. The potentials c1 and ea whose phase is to be compared are each applied to the opposite corners e--! and 47-11, respectively, of the rectifier or modulator bridge.

In Figure 4, the tuned circuits IO, N and I2, I3 are coupled through series coupling condensers l8 replacing the inductances l4 and I5 shown in Figure 1. The operation of this circuit will be more fully understood from the following. Since the inductances l0 and I2 ofier no appreciable resistance for direct current, it may be assumed that terminals e, .f, i and terminals 9,

h, 7' are directly connected together or that the rectifiers I9, 20, 2| and 22 are arranged in parallel to each other with alternate reversal of polarities as far as the direct output potential 1!. is concerned, which direct current component is derived from the center taps of the inductances l0 As a result of this connection or the rectiflers whereby the direct potential developed by the rectifiers 20 and 22 opposes the direct potentials developed by the rectifiers I9 and 2|, the polarity of the output potential set up between the terminals i and a will depend upon the ratio of the amplitudes of the alternating potentials existing across the rectifiers |9, 2| and 20, 22, respectively.

Thus, if in case of extreme detuning the primary potential e1 is in phase with the secondary potential er, the alternating voltage across the rectifier l9, points e and g, as well as that across the rectifier 2 l, points I and h, is smaller than the alternating potential across the rectifier 20,

points h and e, as well as the alternating potential across the rectifier 22, points g and 1. As a result, the potential u between i and 9' is of positive polarity. If, on the other hand, in the case of extreme detuning inthe opposite sense, the potential ez is in counter-phase to the potential 61, the alternating potential across the rectiflers l9 and 2| will be greater than the alternating potential across the rectifiers 20 and 22. As a result, the control potentials produced between i, 7' will be of negative polarity. If the mutual phase for the medium response frequency of the bandpass fllter is 90 in case of resonance, the amplitudes of the alternating potentials will be equal for all rectifiers. As a result, the direct current component in the control potential at will disappear or the control or tuning responsive potential become zero. If inductances are used as coupling elements in place of the condensers l1 and I8 (see Figure 1), the latter should oiier suflicient impedance to direct current to prevent excessive equalizing currents therethrough set up by the output potentialu. V

There is thus produced in the above described manner between terminals 1 and 1' a potential u.

' having a polarity depending upon the sense of band-pass filter relative to an impressed carrier frequency aslong as the tuning remains within predetermined limits.

For large amounts of detuning the mutual phase angle between er and e: varies only slightly in dependence upon the frequency. On the the operating range or limit values of u are governed by the damping of the resonant circuit I0, I and by the degree of coupling or the size of the coupling impedances l4, l5 or H and I8, respectively. The operating range increases for loose coupling and with increased damping of the circuits.

In addition to the direct current component of the potential it responsive to the tuning deviations, the latter also contains harmonics of the potential e1 impressed upon the terminals 0-1; and depending upon the characteristics of the rectifiers. The fundamental frequency is suppressed due to the symmetrical arrangement of the circuit in the case of exact equality of the four rectiflers and by reason of the center tap connections from the inductances l0 and I2 for deriving the tuning responsive potential. y l

The generation of a tuning responsive potential by phase comparison of the primary and secondary potentials by means of a modulator bridge canbe carried out in an analogous manner by employing band-pass filters of the type shown in Figures 2 and 3. An arrangement of this type is shown in Figure 5 which corresponds substantially to Figure 3 and wherein the primary circuit is connected to the corners g-h and the secondary circuit is connected to the corners e-f of the rectifier or modulator bridge, while the output or tuning responsive potential is derived from the center taps of the primary and secondary inductances I0 and I2, respectively, in a manner similarto the preceding exemplification.

Referring to Figure 6, there is shown a similar arrangement employing a band-pass filter comprising a primary tuned circuit III, II and a secondary tuned circuit l2, I3 coupled by an inductive coupling relation between the primary and secondary inductances I ll and 2 and connected to a modulator bridge I9, 20, 2|, 22 in a manner similar to the above.

In arrangements of the aforedescribed type, it is easily seen that the input impedance between terminals aib isdetermined substantially by the tuning and the losses of the primary circuit IO, N. This input impedance is a maximum at or near resonance, that is the energy absorption from an impressed potential e1 is extremely small for slight amounts of detuning. As a result, the damping imposed upon a preceding voltage or potential source or circuit is comparatively small for slight amounts of detuning as is the case in a system for automatic frequency control (AFC).- This fact accounts for the advantage of tuning of the primary circuit.

On the other hand, as is understood, the phase difference between er and e: is in no way affected by the tuning of the primary circuit which latter'accordingly is of no importance as far as the generation of the tune responsive potential by thevphase comparing method is concerned.

Accordingly, the tuning condenser II in Figures 1 I! or 4 may be omitted and a large ohmic or inductive resistance may be provided in place of the tuning inductance ID with a center tap connection for deriving the tune responsive or output potential.

An arrangement of this type is shown in Figure '7 which diifers from Figure 4 by the provision of a high ohmic impedance 23 in place of the primary tuned circuit III, II having a center tap connectedto the output terminal i for deriving the tune responsive potential 11. Also in the case of circuits of the type according to Figure 2, the tuning condenser ll may be omitted in which case the inductive impedance "I should be large relative to the impedance I8 to effect'a phase rotation between er and 8a which is equal to 90 for the average or tuning frequency passed by the filter.

Referring to Figure 8, there is shown a bandpass filter with inductive coupling'of the primary and secondary circuitswherein the primary tuning condenser II is omitted. The circuit otherwise corresponds substantially to the circuit according to Figure 6. Finally also in arrangements according to Figure 5, the inductance I0 may be replaced by an inductive or ohmic impedance with a center tap for deriving a tune responsive or control potential.

According to a further modification of the invention, the tuned secondary circuit l2, l3 may be constructed in the form of a series resonant circuit in place of a parallel resonant circuit shown in the previous exemplifications. In the latter case, the output potential 11. is derived from the center tap of a special impedance permeable to directcurrent and connected in parallel to the tuned circuit.

A circuit of this typ is shown in Figure 9. In the latter a parallel tuned circuit comprising a variable capacity 25 shunted by an inductance 28 is connected in series with a further inductance 21. The input potential e1 is impressed upon the tuned circuit and the series inductance and compared with either of the potentialsez or ea developed across the tuned circuit or series inductance, between terminals m, n or n, I, ,re-

spectively. For the resonance frequency the circuit 25, 26 represents an ohmic or non-reactive impedance in which case the voltage developed across this circuit is phase shifted by 90 compared with the voltage developed across the inductance 21. If detuning occurs the relative phase angle will vary in either direction from 90 in dependence upon the sense and in proportion to the amount of detuning in a manner substantially similar to the circuits shown in the ing 23 having a center tap connection leading to the output terminal 1. the output terminal i being connected to the center tap of the inductance 28 of the parallel tuned circuit. The potential e:

across the secondary 23 has a phase corresponding to the phase of the primary potential across the inductance 21', that is corresponding to the potential es across the reactive impedance 2'! of Figure 9. The output potential it is produced in an analogous manner to the previous arrangements by comparing the phases of the potentials e: and e: by means of the modulator bridge I3, 23, 2|, 22.

Referring to Figure 11 there is shown a further resonant circuit suited for the purpose of the invention. According to this modification a series tuned circuit comprising a variable capacity 33 and inductance 3t is connected across the input terminals (1-4). This circuit offers a pure ohmic resistance to the impressed potential er in case of resonance whereby the potentials ea developed across the inductance 3| or the potential es developed across the capacity 30 are in phase quadrature to the impressed potential e1. In dependence upon the sense and amount of detun- .ing the mutual phase angle will deviate from 90 in a similar manner as in the previous arrangement thereby making it possible to derive an output or control potential by comparing the phases of the potentials es and er or ea and er, respectively.

A complete discriminating system of this type is shown in Figure 12. According to the latter,

the phase of the voltage e2 developed across the inductance 3| is compared with the phase of the impressed potential e1 by mutual modulation in a modulator bridge I3, 20, 2|, 22 to produce a tune responsive or control potential it at the outbodiment a pair of series resonant circuits com-- prising inductances 33 and 33' and capacities 34 and 34' tuned to the same frequency are arranged serially between the input and outputterminals ac and bd, respectively. Since these circuits at resonance represent ohmic resistances, the potential ez developed across a small inductive or capacitative impedance 35 across the output terminals is phase shifted relative to the input potential e1 by 90. relative to the impressed frequency, the relative phase angle will vary in either sense in a manner similar to the preceding arrangements. The potentials c1 and e: may be compared and combined in a rectifier arrangement of the type described hereinbefore and illustrated in Figure .14. In the latter the output potential u is derived from the center tap of the reactive impedance 35 and an additional reactive or non-reactive impedance such as a high ohmic resistance 36 placed across the input terminals aPb as shown. In

place of the series resonant circuits 33, 34 and 33', 33 in Figures 13 and 14 corresponding parallel resonant circuits may be employed in which case the inductances 33 and 33' should offer sufiicient resistance to direct current to avoid a short circuit of the output voltage u.

The rectifiers i3, 20, 2!, 22 may impose an appreciable damping upon the tuned circuits. It is advisable for this reason to employ rectifiers having an impedance for alternating current which remains above a definite minimum during operating conditions. The applied load resistance which the rectifiers present to the applied high If the circuits are detuned.

10 neighborhood of the resonance point.

50 pedances 23 or 36.

One of the input terminals may ordinarily be frequency potentials (such as the primary and secondary potentials e1 and er of the band-pass filter) varies depending on the characteristics of the rectifier used. If this resistance is low, the

5 damping imposed upon the tuned circuits by the rectifiers may become too high so that excessive energy drawn from the preceding amplifiers on the one hand will greatly impair the effective phase rotation depending on the frequency in the In order to increase the load resistance in such cases, it is advisable to place an additional ohmic resistance in series with each of the rectifiers. These ohmic resistances may have an equal value or they may.

15 be adjusted in such a manner as to equalize slight deviations of the resistances presented by the associate rectifiers thereby insuring a favorable balance of the rectifier bridge. By means of these additional resistances, the damping can be re- 20 duced considerably without inadmissibly decreasing the amount of the resulting control potential. It is advisable, however, to keep the ohmic load resistance of the circuit energized by the control potential at a high value relative to the .25 above additional resistances.

The rectifiers employed in connection with the invention may be dry rectifiers preferably having an electrical capacitance as low as possible for. high frequencies. A slight capacity of these rec- -3(l tifiers which cannot be avoided may be compensated in most cases by a corresponding decrease of the tuning capacities. In certain cases valve rectifiers may be employed in place of dry rectifiers such as diode rectifiers with electrically 38 isolated'cathodes. I

The circuits described above are in general of asymmetrical character, that is the terminals of the input potential e1 are electrically equivalent. For this purpose it is desirablethat the input potential as a rule should be symmetrical relative to variable potential of this voltage is applied to the center tap of an inductance placed between the input terminals preferably the tuning inductance [0 according to Figures 4, 5, 6, 10 or a large inductance or impedance as shown in Figures 8 and v u 14, while the other variable potential of this asymmetrical input voltage is applied to one of the input terminals provided for a symmetrical input voltage e1. In this case, a Symmetrical voltage is produced by the inductance ID or imconnected to ground for a fixed reference potential point whereby an asymmetrical output potential is obtained as required in most cases for influencing tuned circuits in accordance with variations'of an inductance or capacity. This control potential should be sufiiciently filtered or smoothened to suppress the high frequency components and to prevent hunting effects in an to automatic frequency control system.

It will be evident from the above that the mvention is not limited to the specific embodiments and methods disclosed herein for illustration but that the principle andnovel concept of the invention are susceptible of numerous variations and modifications coming within the broad scope and spirit of the invention as defined in the appended claims. Thespecification' and drawings are to be regarded accordingly in an illustrative rather than a limiting sense.

I claim: e

1. A frequency variation response circuit comprising a resonant circuit tuned to a predetermined frequency, means for impressing thereon a high frequency potential, 'a rectifier bridge comprising four rectifying elements connected in series in like sense to form a closed c rcuit, circuit means for impressing high frequency potential derived from a predetermined portion of said resonant circuit upon one pair of diagonal points of said bridge, further circuit means for impressing high frequency potential derived from a different predetermined portion of said resonant circuit upon the other pair of diagonal points of said bridge, and an output'circuit connected be= tween points of said first and second circuit means. i

2. A frequency variation response circuit comprising a resonant circuit tuned to a predetermined frequency, means'for impressing thereon a high frequency potential normally having a frequency equal to the resonant frequency of said circuit, means for deriving a pair of high frequency potentials from different predetermined portions of said resonant circuit, whereby the phase relation between the derived potentials varies according to the sense and in proportion to the amount of detuning of said resonant circuit relative to the impressed frequency in respect to a predetermined normal phase angle between the derived potentials when the impressed potential is in tune with said resonant circuit, a rectifierbridge comprising four rectifying elements connected in series in like sense to form a closed circuit, circuit means for impressing one of said derived potentials upon one pair of diagonal points of said bridge, further circuit means for impressing the other of said derived potentials upon the other pair of diagonal points of said bridge, and an output circuit connected between points of said first and second circuit means.

3. A frequency variation response circuit comprising a network having primary and secondary circuits coupled with each other, at least one of said circuits being a resonant circuit tuned to a predetermined frequency, means for impressing a high frequency potential upon said'primary circuit normally having a frequency equal to said predetermined'frequency, a rectifier bridge comprising four rectifier elements connected in series in like sense to form a closed circuit, means for impressing high frequency potential derived from one of said circuits upon one pair of diagonal points of said bridge, further means for impress- -ing high frequency potentials derived from the other of said circuits upon the other pair of diagonal points of said bridge, and an output circuit connected between points of said circuits.

4. A frequency variation response circuit comprising a series network comprising a pair of circuit elements, the first element being constituted by a resonant circuit tuned to a predetermined frequency and the second element being constituted by an impedance, means for impressing a high frequency potential upon said network, a rectifier bridge comprising four rectifier elements connected in series in like sense to form a closed circuit, means for impressing a high frequency potential derived from one of' said elements upon one pair of diagonal points of 'said bridge, means for impressing a high frequency potential derived from the other of said elements upon the other pair of diagonal points of said bridge, and an output circuit connected between points of said elements to derive a resultant potential varying in sense and magnitude according to the sense and amount of detuning of said resonant circuit relative to the frequency of the impressed potential.

5. A frequency variation response circuit comprising a rectifier bridge comprising four rectifying elements connected in series in like sense to form a closed circuit, circuits connected to both pairs of opposite diagonal points of said bridge, means for impressing high frequency potentials of like frequency and varying relative phase relation upon said circuits, and an output circuit connected between points of said first mentioned circuits.

6. A frequency variation response circuit comprising a rectifier bridge comprising four rectifier. elements connected in series in like sense to form a closed circuit, circuits connected to both pairs of diagonal points of said bridge, at least two of said circuits constituting a resonant network tuned to a predetermined band of frequencies, means for impressing a high frequency potential upon said network, and means connected between points of said circuits for deriving a rectified output potential.

'7. A frequency variation response circuit comprising a rectifier bridge comprising four rectifier elements connected in series in like sense to form a closed circuit, circuits connected to both pairs of diagonal points of said bridge, at least one of said circuits being a resonant circuit andcoupled with the other circuit to form a band-pass filter tuned to a predetermined band of frequencies, and an output circuit connected between points of said first mentioned circuits.

8. A frequency variation response circuit comprising a rectifier bridge comprising four rectifier elements connected in series in like sense to form a closed circuit, a pair of circuits each arranged in coupling connection with one pair of diagonal points of said bridge, coupling means between and tuning means for said circuits to form a band-pass filter tuned to a predetermined band of frequencies, means for impressing a high frequency potential upon said band-pass filter, and an output circuit connected between points of said circuits.

GUSTAV GUANELLA. 

